Implantable active medical devices are known to adapt their actions to the measured or calculated value of a parameter representative of the metabolic needs of the wearer of the device (i.e., the patient in which the device is implanted).
In the case of cardiac pacemakers, these systems compete to increase the frequency of stimulation pulses (the pacing rate) when one detects an increasing activity of the patient bearing the device, and to decrease the stimulation frequency until reaching a base value (a base stimulation rate) in the case of a diminution of the patient's activity, particularly during phases of rest of the patient.
EP-A-0 550 293 and its corresponding U.S. Pat. No. 5,330,510 (each commonly assigned to ELA Medical) describes such a device, where a cardiac pacemaker stimulation frequency is enslaved to a parameter calculated from the measure of an acceleration at the level of the trunk of the patient. The device also is known as a rate responsive pacemaker. U.S. Pat. No. 5,330,510 is incorporated herein by reference in its entirety.
The hypothesis of this device is that the "module" of the acceleration at the level of the trunk of the patient is linked to the activity of the patient. The term "module" refers to a control algorithm that provides a control parameter that is functionally related to the activity level of the patient. Thus, under the module, one increases the stimulation pulse frequency when one detects a growing or increasing activity of the patient, and one decreases the frequency, until a base value is reached, in the case of diminution of the patient activity, and particularly during phases of rest of the patient.
The activity sensor for detecting the patient acceleration preferably comprises one, two or three accelerometers whose axes are perpendicular to each other (respectively called sensors "1D", "2D" or "3D").
In the case of a sensor 1D (a unidirectional accelerometer), the accelerometer is oriented to detect acceleration according to an anterior-posterior axis, that is to say perpendicular to the chest. In the case of a sensor 2D or a sensor 3D, accelerometers are disposed in the pacemaker case (also known as a can or housing) in a manner so that one axis (or two of axes depending on the type of sensor used) is situated in the plane corresponding to the large flat side of the pacemaker case. The pacemaker is then implanted in such a manner that its vertical axis is appreciably parallel to the vertical axis of the patient.
Nevertheless, the pacemaker case is able to turn or displace slightly in the body of the patient, as compared to its initial implanted position, so that the axes of sensors can be found offset in relation to the predetermined reference mark by the vertical axis of the wearer, the lateral axis (corresponding appreciably to the axis of patient's shoulders), and the anterior-posterior axis (perpendicular to the two preceding axes and appreciably perpendicular to the trunk of the patient).
To avoid these variations and displacement of axes, that is to say of the displacement of the mark of the sensor with respect to the reference mark of the patient, the aforementioned EP-A-0 550293 and U.S. Pat. No. 5,350,510 proposes a pacemaker of type sensor 2D or sensor 3D in which the stimulation frequency is enslaved to a module of acceleration signals recorded (detected or measured) on each of the axes. The "module" in this embodiment is defined as the square root of the sum of the squares of the instantaneous amplitude provided by the individual accelerometers on each of the two or three axes; this module is constant in a change of orthogonal references, and it is representative of the whole of acceleration collected by the various accelerometers in the different axes, and, therefore, of the activity of the patient.
Actual practice and clinical studies show, however, that signals sensed by accelerometers are not all representative of a created or applied effort by the patient. As a result, the device measures the module of acceleration signals as a parameter that is not always well correlated to the real effort provided by the patient, and therefore to the metabolic needs or demand of the patient. Thus, one notices that the shocks caused by feet striking the ground produce a peak of acceleration not representative of a patient effort.
For this reason, the signal of acceleration on the vertical axis is larger during the descent of a staircase than during the ascent, while the effort developed by the patient is, on the contrary, more intense during the ascent. Similarly, activities of running, due to the fact of the phenomenon of heel shocks on the ground, are sensed by the vertical sensor in a disproportionate manner in relation to activities of walking.
Conversely, the pedalling of a bicycle, which generates few vibrations, is poorly interpreted, and the enslavement of a function to a parameter of acceleration results in an a undervaluation of the effort actually developed by the patient.